ABSTRACT Temperature increases because of climate change are expected to cause expansions at the high latitude margins of species distributions, but, in practice, fragmented landscapes act as barriers to colonization for most species. Understanding how species distributions will shift in response to climate change therefore requires techniques that incorporate the combined effects of climate and landscape-scale habitat availability on colonization rates. We use a metapopulation model (Incidence Function Model, IFM) to test effects of fine-scale habitat use on patterns and rates of range expansion by the butterfly Hesperia comma. At its northern range margin in Britain, this species has increased its breadth of microhabitat use because of climate warming, leading to increased colonization rates. We validated the IFM by reconstructing expansions in five habitat networks between 1982 and 2000, before using it to predict metapopulation dynamics over 100 yr, for three scenarios based on observed changes to habitat use. We define the scenarios as “cold-world” (only hot, south-facing 150–250° hillsides are deemed warm enough), “warm-world” in which 100–300° hillsides can be populated, and “hot-world”, where the background climate is warm enough to enable use of all aspects (as increasingly observed). In the simulations, increased habitat availability in the hot-world scenario led to faster range expansion rates, and to long-term differences in distribution size and pattern. Thus, fine-scale changes in the distribution of suitable microclimates led to landscape-scale changes in population size and colonization rate, resulting in coarse-scale changes to the species distribution. Despite use of a wider range of habitats associated with climate change, H. comma is still expected to occupy a small fraction of available habitat in 100 yr. The research shows that metapopulation models represent a potential framework to identify barriers to range expansion, and to predict the effects of environmental change or conservation interventions on species distributions and persistence.

[Show abstract][Hide abstract]ABSTRACT:
Weather extremes may have strong effects on biodiversity, as known from theoretical and modelling studies. Predicted negative effects of increased weather variation are found only for a few species, mostly plants and birds in empirical studies. Therefore, we investigated correlations between weather variability and patterns in occupancy, local colonisations and local extinctions (metapopulation metrics) across four groups of ectotherms: Odonata, Orthoptera, Lepidoptera, and Reptilia. We analysed data of 134 species on a 1×1 km-grid base, collected in the last 20 years from the Netherlands, combining standardised data and opportunistic data. We applied dynamic site-occupancy models and used the results as input for analyses of (i) trends in distribution patterns, (ii) the effect of temperature on colonisation and persistence probability, and (iii) the effect of years with extreme weather on all the three metapopulation metrics. All groups, except butterflies, showed more positive than negative trends in metapopulation metrics. We did not find evidence that the probability of colonisation or persistence increases with temperature nor that extreme weather events are reflected in higher extinction risks. We could not prove that weather extremes have visible and consistent negative effects on ectothermic species in temperate northern hemisphere. These findings do not confirm the general prediction that increased weather variability imperils biodiversity. We conclude that weather extremes might not be ecologically relevant for the majority of species. Populations might be buffered against weather variation (e.g. by habitat heterogeneity), or other factors might be masking the effects (e.g. availability and quality of habitat). Consequently, we postulate that weather extremes have less, or different, impact in real world metapopulations than theory and models suggest.

[Show abstract][Hide abstract]ABSTRACT:
The climate change risk to biodiversity operates alongside a range of anthropogenic pressures. These include habitat loss and fragmentation, which may prevent species from migrating between isolated habitat patches in order to track their suitable climate space. Predictive modelling has advanced in scope and complexity to integrate: (i) projected shifts in climate suitability, with (ii) spatial patterns of landscape habitat quality and rates of dispersal. This improved ecological realism is suited to data-rich model species, though its broader generalisation comes with accumulated uncertainties, e.g. incomplete knowledge of species response to variable habitat quality, parameterisation of dispersal kernels etc. This study adopts ancient woodland indicator species (lichen epiphytes) as a guild that couples relative simplicity with biological rigour. Subjectively-assigned indicator species were statistically tested against a binary habitat map of woodlands of known continuity (>250 yr), and bioclimatic models were used to demonstrate trends in their increased/decreased environmental suitability under conditions of ‘no dispersal’. Given the expectation of rapid climate change on ecological time-scales, no dispersal for ancient woodland indicators becomes a plausible assumption. The risk to ancient woodland indicators is spatially structured (greater in a relative continental compared to an oceanic climatic zone), though regional differences are weakened by significant variation (within regions) in woodland extent. As a corollary, ancient woodland indicators that are sensitive to projected climate change scenarios may be excellent targets for monitoring climate change impacts for biodiversity at a site-scale, including the outcome of strategic habitat management (climate change adaptation) designed to offset risk for dispersal-limited species.

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Linking habitat use to range expansion rates in fragmentedlandscapes: a metapopulation approachRobert J. Wilson, Zoe G. Davies and Chris D. ThomasR. J. Wilson (R.J.Wilson@exeter.ac.uk), Centre for Ecology and Conservation, Univ. of Exeter, Cornwall Campus, Penryn TR10 9EZ, UK.? Z. G. Davies, Dept of Animal and Plant Sciences, Univ. of Sheffield, Sheffield S10 2TN, UK. ? C. D. Thomas, Dept of Biology, Univ. ofYork, York YO10 5YW, UK.Temperature increases because of climate change are expected to cause expansions at the high latitude margins of speciesdistributions, but, in practice, fragmented landscapes act as barriers to colonization for most species. Understanding howspecies distributions will shift in response to climate change therefore requires techniques that incorporate the combinedeffects of climate and landscape-scale habitat availability on colonization rates. We use a metapopulation model(Incidence Function Model, IFM) to test effects of fine-scale habitat use on patterns and rates of range expansion by thebutterfly Hesperia comma. At its northern range margin in Britain, this species has increased its breadth of microhabitatuse because of climate warming, leading to increased colonization rates. We validated the IFM by reconstructingexpansions in five habitat networks between 1982 and 2000, before using it to predict metapopulation dynamics over100 yr, for three scenarios based on observed changes to habitat use. We define the scenarios as ‘‘cold-world’’ (only hot,south-facing 150?2508 hillsides are deemed warm enough), ‘‘warm-world’’ in which 100?3008 hillsides can bepopulated, and ‘‘hot-world’’, where the background climate is warm enough to enable use of all aspects (as increasinglyobserved). In the simulations, increased habitat availability in the hot-world scenario led to faster range expansion rates,and to long-term differences in distribution size and pattern. Thus, fine-scale changes in the distribution of suitablemicroclimates led to landscape-scale changes in population size and colonization rate, resulting in coarse-scale changes tothe species distribution. Despite use of a wider range of habitats associated with climate change, H. comma is still expectedto occupy a small fraction of available habitat in 100 yr. The research shows that metapopulation models represent apotential framework to identify barriers to range expansion, and to predict the effects of environmental change orconservation interventions on species distributions and persistence.The capacity of species to survive climate change willdepend on their ability either to adapt in situ to changingconditions, or to colonize regions that become suitableoutside their current geographic range. Some bioclimatemodels predict that future climatically-suitable locationsmay show little or no overlap with the current ranges ofmany species (Thomas et al. 2004, Fitzpatrick et al. 2008).Despite reservations regarding the accuracy of such models(Beale et al. 2008, Trivedi et al. 2008, Kriticos and Lerichepers. comm.), a wide range of models (Arau ´jo et al. 2005,Elith et al. 2006) and empirically observed species rangeshifts (Parmesan 2006) suggest that the conservation ofmany species will depend on their ability to cross landscapesthat have been heavily modified by human activities.Current evidence suggests that a number of habitat general-ist or dispersive species have been able to expand theirdistributions polewards associated with climate change(Warren et al. 2001), but that most species (particularlyhabitat specialists) are failing to colonize climaticallysuitable regions as they become available (Mene ´ndez et al.2006). Indeed, the distributions of many taxa appear notto have fully occupied available climate space during thepresent interglacial (Arau ´jo and Pearson 2005, Svenningand Skov 2007, Arau ´jo et al. 2008, Svenning et al. 2008).To aid the conservation of such species in a changingclimate, methods are required not only to predict thedistribution of future suitable locations (Arau ´jo and New2007, Beaumont et al. 2007), but also the species traits(Ward and Masters 2007, Massot et al. 2008) andlandscape or habitat features that will facilitate speciesrange expansions (Hill et al. 2001, Allouche et al. 2008,Vos et al. 2008).One important consequence of climate change is thatspecies’ regional habitat use may change (Davies et al.2006). Many species are restricted to hot microhabitats attheir high latitude range margins, but occupy a broaderspectrum of microhabitats nearer the centres of their ranges(Thomas 1993, Thomas et al. 1999). For such species,climate change might be expected to increase the breadth ofhabitat requirements towards high latitudes, hence increas-ing habitat availability, population sizes and colonizationrates (Thomas et al. 2001). The important question forEcography 33: 73?82, 2010doi: 10.1111/j.1600-0587.2009.06038.x# 2010 The Authors. Journal compilation # 2010 EcographySubject Editors: Nu ´ria Roura-Pascual and Nathan Sanders. Accepted 10 September 200973S P E C I A LI S S U E

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conservationists is whether the increases in habitat avail-ability owing to broader habitat use will be sufficient toallow species to expand their distributions through frag-mented landscapes.In this paper we use a metapopulation model to test howchanges to habitat use because of climate change affect aspecies’ distribution at its upper latitude margin. In Britain,the silver-spotted skipper butterfly Hesperia comma isrestricted to hot microhabitats in calcareous grassland(Thomas et al. 1986), but its breadth of habitat use hasrecently expanded due to changed environmental condi-tions (warmer summers; Davies et al. 2006). We show howthese changes to habitat use increase habitat availability,before using the Incidence Function Model (Hanski 1994,1999) to simulate how increased habitat area and con-nectivity lead to faster rates of range expansion, and to long-term differences in distribution size and pattern. In thissystem, colonization and extinction dynamics provide amechanism linking fine-scale changes in habitat use toregional-scale changes in species distributions.Materials and methodsStudy systemIn Britain, the silver-spotted skipper butterfly Hesperiacomma is restricted to short-turfed chalk grassland, where itlays eggs exclusively on short tufts (B10 cm) of sheep’sfescue grass Festuca ovina (Thomas et al. 1986). During thetwentieth century, agricultural intensification, the abandon-ment of low intensity livestock grazing, and the virtualelimination of rabbits because of myxomatosis led to adramatic reduction in habitat availability for the species.When a full survey of the British distribution of H. commawas undertaken in 1982, it had declined to fewer than 70populations (Thomas et al. 1986). The vast majority ofthese refuge populations were in five habitat networks insouth-east England (Surrey, Sussex, Chilterns, Kent, Hamp-shire; Fig. 1), and on south-facing slopes (90% on aspects of100?3008; 75% on aspects of 150?2508). At the time, thespecies appeared to select the hottest microhabitats foroviposition, with an estimated optimum sward compositionincluding 41% bare ground (Thomas et al. 1986). In 1991,repeat distribution surveys in Surrey and Sussex showed thata range expansion had begun in Sussex, and that themaximum colonization event was 8.65 km from the nearest1982 population (Thomas and Jones 1993).We carried out a repeat survey of the distribution ofH. comma in 2000, searching for adults and eggs of thespecies in all areas of chalk grassland containing suitableF. ovina plants within 30 km of the 1982 H. commadistribution, and within 10?15 km of newly-colonized sitesfound in 2000. Such a comprehensive search was feasiblebecause suitable habitat occurs only on unimprovedcalcareous grassland, which is restricted to localized chalkescarpments in south-east England (Fig. 1). We definedhabitat patches as areas of suitable grassland bounded bycontinuous woodland or scrub barriers, or by at least 25 mof unsuitable grassland (Thomas and Jones 1993, Hill et al.1996), and recorded the area, aspect (8) and vegetationcomposition of each habitat patch, and of subdivisions ofhabitat patches where contiguous habitats or aspects variedmarkedly (Davies et al. 2005). Since 1982, livestock grazinghas been reintroduced to many areas of chalk grassland aspart of conservation programmes, and rabbit numbers haveincreased (Davies et al. 2005). In addition, the temperatureduring H. comma’s flight period increased by 2.88C over20 yr as a result of direct temperature changes andphenological advancement of the butterfly’s flight period(Wilson et al. 2007). At higher ambient temperatures,H. comma oviposition rate increases, and oviposition sitesare less restricted to hot microclimates (Davies et al. 2006).As a result, optimum sward composition for H. comma egg-laying in 2001 and 2002 included a reduced cover of bareground (21%), and by 2000 the species had colonizednorth-, east- and west-facing aspects, where bare groundcover and ambient temperature are typically lower than onsouth-facing slopes (Thomas et al. 2001, Davies et al.2006). In conjunction with these changes, the speciesexpanded its distribution in Britain, colonizing 179 habitatpatches between 1982 and 2000 (Davies et al. 2005).Nevertheless, large areas of suitable habitat remainedunoccupied (Fig. 1). In 2002, intensive distribution surveyswere again repeated in the Surrey and Sussex networks,showing a limited number of colonization events since2000.The metapopulation modelThe Incidence Function Model (IFM) (Hanski 1994) is astochastic patch occupancy model based on the assumptionsthat 1) extinction risk is inversely related to patch area orpopulation size, and 2) colonization probability is positivelyrelated to patch connectivity, where connectivity is afunction of the distance to other occupied patches andtheir size. For each patch i, annual extinction risk (Ei) isrelated to patch area (Ai) by the term Ei?(e/ /Axwhere e and x are species-specific parameters relatingextinction risk to area, and (1?Ci) takes account ofthe rescue effect by reducing extinction risk for highlyi) (1?Ci),HampshireSurreyChilternsKentSussexN50 kmFigure 1. The distribution of Hesperia comma in 1982 and 2000in south-east England. Symbols show occupancy status of habitatpatches in five networks: survived (black, occupied 1982 and2000); colonized (grey, absent 1982, occupied 2000); extinct (greytriangles, occupied 1982, absent 2000); vacant (white, absent 1982and 2000). Grey squares show populations introduced between1982 and 2000. Symbol size exaggerates patch area: smallB0.1 ha, medium 0.1?1 ha, large ?1 ha. Line indicates the coast.74S P E C I A LI S S U E

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connected patches. Annual colonization probability (Ci) isrelated to connectivity (Si) by the term Ci? /S2where y is a species-specific parameter relating colonizationrate to connectivity. Connectivity is estimated as Si?a exp(?adij) Aba negative exponential dispersal kernel, with the proportionof per generation dispersal over distance d km or greatercorresponding to exp?ad; dijis the distance to patch i fromeach other patch j; Ajis the area of each patch j; and b scalesemigration rate to the area of patch j. In this study b was setto 0.5 to account for the tendency of per capita emigrationto be greater from smaller habitat patches in this and otherspecies (Hill et al. 1996, Moilanen and Nieminen 2002).The IFM carries out maximum likelihood estimation ofparameters e, x, y and a for a species based on snapshots ofoccupied and vacant habitat patches (Moilanen 1999,2000). The model was developed to estimate parametersfrom steady state metapopulations, where observed occu-pancy is considered to result from approximately equalcolonization and extinction rates, although parameters mayalso be estimated from population turnover withoutassuming equilibrium dynamics (Moilanen 2000). Theestimated parameters can then be used to simulate dynamicsfor the species in the same or other, potentially non-equilibrium metapopulations, which may be valuable forpredicting changes to species distributions following envir-onmental change (Thomas and Hanski 2004).A metapopulation model is an appropriate frameworkfor modelling H. comma’s dynamics in Britain because 1) ithas clearly defined, localized habitat patches; 2) mostpopulations are small and at some risk of extinction, withoccasional extinctions in 1982?2000 despite generallyincreasing population sizes (Thomas and Jones 1993,Davies et al. 2005); 3) there is limited dispersal betweenH. comma populations (Hill et al. 1996), and colonizationrates decrease with distance from populations (Thomas andJones 1993, Davies et al. 2005); 4) the probability ofhabitat patch occupancy increases with patch area andconnectivity to other populations of the species (Thomaset al. 1992); 5) the distribution of the species in onepopulation network in the UK (Surrey) remained ratherstable between 1982 and 2002 (Davies et al. 2005),allowing us to estimate metapopulation parameters forthis network assuming a steady state.For IFM parameter estimation we used the Monte CarloMarkov Chain method (1000 function evaluations ininitiation, 4000 function evaluations in estimation). Weassumed, based on field observations, that the minimumarea which would support a population from one year to thenext without immigration (A0) was 0.02 ha. For thisanalysis, we use the parameter estimates of x?0.28, y?7.26, e?0.34, a?0.45 (Thomas et al. 2001) assumingthat the distribution of the species in Surrey in 2000 (86occupied and 30 vacant habitat patches) was representativeof a stochastic steady state. We also estimated parametersusing information on turnover in Surrey in 1982, 1991,2000 and 2002, not assuming a stochastic steady state(Moilanen 2000). Simulations using these parameters (x?0.39, y?9.44, e?0.22, a?0.45) are not described indetail since they gave generally similar results to theparameters assuming a steady state in Surrey.i/( /S2i?y2),j(Moilanen and Nieminen 2002). Parameter a isMetapopulation simulationsWe tested the effects of habitat use on H. comma rangeexpansion rates by simulating metapopulation dynamicsthrough habitat networks based on three different scenariosof habitat availability. In a thermally-restricted or ‘‘cold-world’’ scenario: 1) only areas of habitat with aspectsbetween 1508 and 2508 were included, corresponding to75% of occupied habitat patches in 1982. In a ‘‘warm-world’’ habitat definition, broadly corresponding to thatused in 1982, 2) aspects between 1008 and 3008 wereincluded, corresponding to 90% of populations in 1982. Inthe broadest or ‘‘hot-world’’ habitat definition, correspond-ing more closely to habitat use in 2000, 3) areas of chalkgrassland on all aspects were included in the simulations.We first validated the model by running 100 IFMsimulations of 18 yr using the parameters estimated fromthe Surrey network, starting with the distribution of thespecies in each of the five habitat networks in 1982. To testwhether the model successfully predicted the relative like-lihood that different patches would be occupied in 2000, weused the area under the curve (AUC) of the receiveroperating characteristic, a method which allows testing ofthe significance with which modelled probabilities correctlyclassify cases (Fielding and Bell 1997). In this case, wecalculated whether the proportion of simulations whereeach patch was occupied was significantly related toobserved presence or absence of H. comma in 2000 foreach network using SPSS v. 15.0 for Windows. To testwhether the model accurately predicted distance expanded,we calculated for each network the mean distance tocolonized patches in 2000 from the nearest 1982 popula-tion. We then calculated the same measure for each 18 yrsimulation (from 1982 to 2000), and tested whetherobserved range expansions lay outside the 95% confidenceintervals (2.5th to 97.5th percentiles) of the modelledexpansions. In order to test the longer term consequences ofhabitat use for the range dynamics of H. comma, we thensimulated metapopulation dynamics for 100 yr for the threehabitat use scenarios and five habitat networks. To testwhether the distribution pattern used to initiate metapo-pulation simulations would influence long-term modelleddynamics, we started simulations both from a) the speciesdistribution in 1982, and b) the distribution in 2000.ResultsNetwork habitat availabilityIncluding habitat from all aspects, we identified 1916.6 haof suitable habitat for H. comma in the five networks(Table 1a). Overall, the area of habitat including all aspects(the ‘‘hot-world’’ scenario) was 1.4 times greater thanhabitat availability for the ‘‘warm-world’’ scenario of 100?3008, and 2.1 times greater than habitat availability for the‘‘cold-world’’ scenario of 150?2508. Changes in patchnumbers were similar to changes in habitat area: the totalnumber of suitable patches for all aspects was 1.3 times thatfor the 100?3008 scenario, and double that for the 150?2508 scenario. However, the difference between the threehabitat scenarios differed markedly among networks: there75S P E C I A LI S S U E

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was little difference in habitat availability between scenariosfor the predominantly south-facing Surrey network (mostsites would already have been sufficiently warm in 1982),whereas in the two networks on the largely north-facingSouth Downs escarpment (Sussex and Hampshire), habitatavailability for the 0?3608 scenario was ?3.5 times greaterthan for the 150?2508 scenario (Table 1a).Model validation 1982?2000Simulations gave good predictions of the relative likelihoodthat individual patches would be occupied in 2000 in eachnetwork, with AUC values of 0.85?0.90 for the 0?3608networks, or of 0.73?0.88 for 100?3008 networks (Table2). AUC values were higher for the 0?3608 scenario thanfor the 100?3008 scenario in all networks except Kent,although differences were small.The observed mean distance to each patch colonized by2000 from the nearest patch that was occupied in 1982ranged from 1.3 km in Surrey to 5.8 km in Sussex(Table 2). Observed mean colonization distances lay withinthe 95% confidence intervals for IFM simulations using the0?3608 habitat scenario for Surrey, Sussex and Hampshire,but were overestimated for Kent and underestimated for theChilterns (Table 2). Simulations using the 100?3008habitat scenario underestimated range expansions forSussex, Hampshire, and the Chilterns, and overestimatedexpansion in Kent. Colonization distance was accuratelypredicted in Surrey, for both 100?3008 and 0?3608scenarios, but this was not surprising because in Surreythe 0?3608 scenario was only 1% greater in area than the100?3008 scenario. In contrast, modelled distances ex-panded over 18 yr were greatly underestimated by the 100?3008 scenario in Hampshire and Sussex, where 40 and 51%respectively of potential habitat lay outside the 100?3008range of aspects. Comparison of modelled occupancy inSussex with that observed after 9, 18 and 20 yr from 1982suggests that the 100?3008 habitat scenario gave ratheraccurate predictions of occupancy after 9 yr (1991; Fig. 2b),but that by 2000 (18 yr) and 2002 (20 yr) inclusion of 0?3608 habitat was necessary to produce accurate predictionsof occupancy (Fig. 2a). The overestimation of colonizationdistance in Kent appears to be related to a lower percentagecover of larval host plant than in the other networks, and ifthis is taken into account then observed colonizationdistances fall within the 95% confidence intervals for 0?3608 simulations (Wilson et al. 2009).100 year metapopulation simulationsWhether simulations were started with the 1982 or 2000observed distributions of populations, each network showeda lower predicted proportion of patches occupied after100 yr for the 150?2508 habitat scenario than for the 100?3008 scenario, and for the 100?3008 scenario versus the 0?3608 scenario (Table 1b). These differences were significantin all cases (Mann-Whitney U tests for 100 simulations ineach scenario, pB0.001), apart from 150?2508 versus100?3008 simulations from the 1982 distribution inHampshire, where most simulations suffered extinction inB100 yr. The difference was particularly marked for Sussex,a landscape that experiences a major increase in habitatavailability with climate warming: on average only 2?3% ofpatches were occupied after 100 yr for the 150?2508simulations, 24?28% were occupied in the 100?3008simulations, and 67?70% were occupied in the 0?3608simulations (Table 1b, Fig. 2). In Sussex, the averagenumber of patches that were occupied after 100 yr in the 0?3608 scenario was more than five times that for the 100?3008 scenario (Fig. 2).Four of the habitat networks also showed significantdifferences in simulated occupancy after 100 yr dependingon the starting distribution (1982 versus 2000). Thisdifference appears to result from 1) the higher risk ofextinction of 1982 metapopulations, which had fewerpopulations than in 2000; 2) some observed long-distancecolonizations between 1982 and 2000 which led topopulation networks predicted to be persistent in thelong-term. Sussex, Hampshire and the Chilterns all showedsignificantly higher occupancy when started with the 2000distribution, both for 0?3608 and 100?3008 scenarios(Mann-Whitney U tests for 100 simulations in eachTable 1. Habitat availability and modelled patch occupancy in the five networks under the three habitat use scenarios: ‘‘hot-world’’ (0?3608aspect), ‘‘warm-world’’ (100?3008) and ‘‘cold-world’’ (150?2508). a) Total habitat area (ha), number of patches in superscript; b) meanproportion of patches occupied after 100 metapopulation simulations of 100 yr starting from the 1982 distribution of H. comma (superscriptshows patch occupancy starting from the 2000 distribution; values in bold show significant differences from simulations initiated in 1982).NetworkHabitat scenario0?3608100?3008150?2508a) Total habitat area (ha)(n patches)SurreySussexChilternsKentHampshireTotal178.7(116)713.7(212)286.7(161)457.7(136)279.8(117)1916.6(742)176.9(113)349.3(123)251.1(135)386.8(110)168.1(72)1332.1(553)151.9(95)199.5(67)166.0(90)302.0(70)79.6(35)898.9(357)b) Proportion patch occupancy after 100 yr (start 1982)(start 2000)SurreySussexChilternsKentHampshire0.74(0.74)0.67(0.70)0.25(0.31)0.53(0.55)0.22(0.47)0.73(0.72)0.24(0.28)0.10(0.13)0.43(0.43)0.01(0.28)0.64(0.65)0.02(0.03)0.01(0.01)0.29(0.31)0.00(0.00)76S P E C I A LI S S U E

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scenario, pB0.01). Kent showed a significant difference forthe 0?3608 scenario (pB0.001), whereas Surrey showed nosignificant differences in occupancy related to the startingdistribution, probably because the butterfly was alreadywidespread and had limited capacity to spread in thislandscape, and can fully exploit the remaining patches fromeither starting scenario. For the 150?2508 simulations,occupancy did not differ depending on starting conditions,apart from in Sussex where occupancy was marginallyhigher when starting from 2000 (p?0.02).Simulated UK distributions for H. comma after 100 yrare shown in Fig. 3, based on 2?2 km distribution‘‘tetrads’’ (commonly used in regional species distributionmaps). In total, 287 tetrads with suitable habitat wereidentified in the five networks, of which 26 were occupiedin 1982 and 96 were occupied in 2000. For the 0?3608habitat scenario, H. comma’s distribution was predicted toinclude 136 tetrads after 100 yr (occupied in 50% or moreof simulations) when starting with the 1982 distribution, or164 tetrads when starting with the 2000 distribution. The100?3008 scenario led to a distribution size after 100 yr of61 tetrads (start 1982) or 81 tetrads (start 2000). The 150?2508 scenario led to a predicted distribution size of 31tetrads when starting with either distribution: for thisscenario, the networks in Sussex, Hampshire and theChilterns were expected to suffer extinction in ?50% ofsimulations (e.g. Fig. 2c, f for Sussex).DiscussionHere we show how changes in habitat use and availability,as may occur in response to climate change, can influencerates of range expansion through fragmented landscapes.The results shed light on the processes that govern the sizeand pattern of species distributions, and have implicationsfor the conservation of localized species in a changingclimate.Habitat availability and metapopulation dynamicsWe studied the habitat use, distribution and dynamics ofthe butterfly Hesperia comma in its five main UK popula-tion networks. The breadth of habitats used by H. commaincreased between 1982 and 2000 in association withclimate warming (Thomas et al. 2001, Davies et al.2006). Habitat area in the five networks increased by1.4 times (and the number of patches by 1.3 times) becauseof an increase in the range of aspects used, from 100?3008in 1982 to 0?3608 in 2000. Greater habitat availabilityshould lead to lower rates of local extinction due to largerpopulation sizes; and to higher rates of colonization,because areas of suitable habitat are closer together, andmore and larger populations are available to act as sources ofcolonists. To model these effects, we applied the IncidenceFunction Model (IFM) (Hanski 1994, 1999) to thedynamics of H. comma. The model gave a relatively goodfit to patterns and rates of range expansion between 1982and 2000, capturing the differences in colonization dis-tances among networks varying in habitat area andconnectivity (see also Wilson et al. 2009). Furthermore,Table 2. Validation of metapopulation simulations against observed distribution change in 1982?2000. Average expansion distance is the mean distance to colonized patches from the nearest patchoccupied in 1982. Modelled expansions show the median and 95% confidence intervals of 100 Incidence Function Model (IFM) 18 yr simulations; 95% CIs of modelled expansion are shown in boldwhere they include the observed expansion distance. AUC is calculated for observed patch occupancy in 2000 against proportion occupancy in the 100 simulations.Patch occupancy, 2000Average expansion distance (km)AUC of modelled patch occupancyn presentn absentObserved distanceIFM 0?3608IFM 100?3008IFM 0?3608IFM 100?3008Surrey86301.330.95 (0.65?1.49)1.27 (0.71?1.84)0.88***0.87***Sussex79965.806.21 (4.48?7.21)2.38 (0?2.93)0.87***0.83***Chilterns441173.851.93 (0.96?3.41)1.32 (0.60?2.30)0.86***0.86***Kent271023.565.42 (4.86?5.98)5.20 (3.69?5.97)0.85***0.88*Hampshire121055.436.10 (0.22?8.64)0 (0?2.18)0.90***0.73*77S P E C I A LI S S U E

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the model showed how rates of range expansion acceleratedthrough H. comma’s population networks associated withwider habitat availability, as demonstrated by the under-estimation of empirically observed expansion rates bymetapopulation simulations using the 100?3008 habitatscenario (Table 2). Our estimated changes to habitat areaassume that all aspects were managed appropriately forH. comma in 1982 and 2000: in practice, much habitatin 1982 was not managed appropriately for H. comma,and subsequent introduction of grazing management as partof conservation programmes and agri-environment schemeshas been necessary for the species to exploit potentialhabitats (Davies et al. 2005). These changes to habitatmanagement may partly explain why observed expansionrate in Sussex in 1991 was overestimated by the simulations(Fig. 2a).The consequences of greater habitat availability andfaster range expansion rates for long-term distributionpatterns were investigated using 100 yr metapopulationsimulations, from H. comma’s distribution either in 1982 or2000. Broader habitat use led to a higher proportion ofpatches occupied after 100 yr (ca 1.5 times more for 0?3608versus the 100?3008 scenario; Table 1), which equated toapproximately double the number of patches occupied,and double the overall number of 2?2 km ‘‘tetrads’’ inH. comma’s distribution (Fig. 3). Thus, metapopulationdynamics provided the mechanism linking changes inhabitat use to relatively large-scale changes to the speciesdistribution.Habitat fragmentation and species range shiftsDispersal failure is potentially hugely important in deter-mining the consequences of climate change for biodiversity.The complete failure of species to expand their distributionsStart 2000 Start 19820°–360° 02550751001251501750255075100125150175100°–300° 02550751000255075100150°–250° Number of patches occupied 02040600204060020406080 100020406080 100020406080 100020406080100020406080100Time (years) 020406080100(a) (b) (c) (d) (e) (f) Figure 2. Modelled patch occupancy over 100 yr in Sussex based on different habitat availability and starting distributions. Panels a?cshow simulations starting from the 1982 distribution; d?f show simulations from the 2000 distribution. Habitat availability includes allavailable aspects (a, d), 100?3008 aspects (b, e), or 150?2508 aspects (c, f). Solid line shows median patch occupancy from 100 IFMsimulations, dashed lines show 95% confidence intervals (lower interval reaches zero occupancy in b, c and f). Diamonds in panels a and bshow observed occupancy in 1991 (9 yr since 1982), 2000 (18 yr) and 2002 (20 yr).78S P E C I A LI S S U E

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into new regions could approximately double extinctionrates from climate change, relative to rates that assume fullcolonization (Thomas et al. 2004). Research on butterflies(Warren et al. 2001, Mene ´ndez et al. 2006) and birds(Julliard et al. 2004) has already shown that more wide-ranging or habitat generalist species have been able to shifttheir distributions in response to climate change much morereadily than sedentary species with localized habitatdistributions. Our study species H. comma is a habitat-specialist that has been able to expand its distribution(Thomas et al. 2001, Davies et al. 2006). However, despiteincreasing habitat availability because of climate change,conservation management and agri-environment schemes(Davies et al. 2005), H. comma’s range expansion rates havebeen constrained by habitat fragmentation (Wilson et al.2009). In this paper, we show that even in the long term thespecies is only expected to colonize a fraction of its availablehabitat. The most optimistic ‘‘hot-world’’ metapopulationsimulations (with the broadest habitat requirements) sug-gest that after 100 yr only about a half of existing suitablehabitat patches would be occupied by the species (49% ifstarting with the 1982 distribution; 56% if starting with the2000 distribution; or 47% of 2?2 km tetrads containingsuitable habitat starting from 1982, versus 57% startingfrom 2000). Should regional habitat requirements becomenarrower, then the proportion of suitable habitat occupiedby the species would likewise be expected to reduce (Fig. 3).Our results should be considered as indicative of thepotential differences in realized range expansions amonglandscapes differing in topography (slope and aspect) andlevel of habitat fragmentation, rather than as absolutepredictions of range expansion rates. It has been shownthat insect dispersal rates may increase related to warmertemperatures (Nieminen 1996, Sparks et al. 2005), poten-tially leading to faster colonization rates. Furthermore,increased dispersal ability may be selected for in newlyestablished populations (Hanski et al. 2006) and at theexpanding front of species distributions (Niemela andSpence 1991, Thomas et al. 2001, Simmons and Thomas2004). In the case of H. comma, individuals in theexpanding Sussex network show larger thorax:abdomensize ratios than in the more stable Surrey network (Hill et al.1999), suggesting that more dispersive forms may indeedhave been selected for by the process of range expansion.Nevertheless, parameters estimated from the Surrey net-work gave relatively good fits to expansion rates andpatterns in the other metapopulations (Wilson et al.2009). Habitat availability at the landscape scale may stillbe critical in explaining range expansion rates, since asufficient density of habitat may be necessary to promotecolonization in the first place, and therefore to select fordispersive forms.The differences among the five networks in observed andmodelled rates of range expansion, and in future predicted 0002 t ra tS 289 1 t ra tS0°–360° 50 kmN50 kmN100°–300° 50 kmN50 kmN150°–250° 50 kmN50 kmN

distributions under the three habitat scenarios, show thatthere is an important landscape context to species responsesto climate change. The metapopulation in one landscape(Surrey) was persistent under all three habitat scenarios, andshowed no significant differences in long term distributionpatterns related to the distribution used to initiate simula-tions (1982 versus 2000). In contrast, the Sussex networkshowed significant differences in long term occupancyamong all three habitat scenarios, and between 1982 and2000 starting conditions; while the Hampshire metapopu-lation was only predicted to be persistent for 0?3608 and100?3008 scenarios started from 2000. There is a relativelysmall proportion of habitat in Sussex and Hampshire onsouth-facing slopes, hence the ability to use a wider range ofaspects has been critical in allowing H. comma to expand itsdistribution from the regional refuge populations (probablyone population in each network in the late 1970s or early1980s; Thomas et al. 1986). In contrast, the majority ofhabitat in the Surrey network is south-facing; persistence inthis network seems likely, as long as environmental changedoes not render south-facing slopes unsuitable. Moreover,the proportion of habitat occupied by H. comma remainsrelatively high in Surrey, because although the overall areaof habitat is quite small, patches are densely distributed overa relatively restricted area.Two scenarios could lead to metapopulation declines inSurrey: reduced habitat suitability if south-facing slopesbecome too hot or dry under climate change, or a reductionin suitable warm microclimates for larval growth because ofincreasing vegetative growth related to longer growingseasons or nitrogen deposition (WallisDeVries and VanSwaay 2006). In fact, south-facing slopes may be relativelyresistant to the latter type of vegetation change because offrequent exposure to drought conditions (Bennie et al.2006). Simulations suggest that the Surrey metapopulationmay be unlikely to expand because of a low density ofsuitable habitat at its eastern and western extremes (Fig. 1,3). Nevertheless, a site at the eastern edge of the Surreynetwork (11.4 km from the nearest 1982 population) wascolonized in 2002, suggesting that there is scope for furtherexpansion in this landscape. Metapopulation simulationsusing parameters derived from the Surrey network shouldbe treated with some caution since they rely on theassumption that the Surrey network in 2000 was atcolonization/extinction equilibrium. Furthermore, the un-certain future interactions between climate, habitat condi-tion and habitat use mean that long-term changes to speciesdistributions may be very difficult to predict with anydegree of certainty.Simulations suggest that in many fragmented landscapes,expanding metapopulations do not approach equilibriagradually (e.g. for Sussex see Fig. 2a, d). Instead, highoccupancy is achieved quickly among habitat patches closeto the starting distribution. Subsequently, relatively rarelong-distance colonization events result in colonization ofother persistent habitat sub-networks which can achievehigh levels of occupancy, leading to what appear as step-changes in metapopulation occupancy (e.g. Fig. 2a, step-changes ca 20 yr and ca 60 yr into the simulation). Suchrelatively rare colonization events can be important indetermining large-scale species distributions, and probablyplay a key role in explaining why simulations started fromthe 2000 distribution achieved greater patch occupancythan those started from the more restricted 1982 distribu-tion. Indeed, there is a risk that models such as that usedhere might underestimate rates of range expansion becauseof the role played by rare long-distance colonization events.Refuge populations for species may reflect historicalclimate or habitat requirements (Petit and Burel 1998,Helm et al. 2006), and may not necessarily represent themost efficient locations for seeding range expansions intohabitats which are suitable now or in the future. Thedifferences in simulation outcomes starting from 1982versus 2000 distributions show that historical factors caninfluence long-term distribution size and pattern, and couldconstrain conservation efforts to facilitate species rangeshifts in response to climate change. In Sussex, for example,the observed 1982?2000 range expansion resulted inH. comma colonizing a part of the habitat-network that ispersistent under the 100?3008 scenario (Fig. 3e). As aresult, 100?3008 simulations started with the 2000 dis-tribution achieved significantly higher occupancy than thosestarted with the 1982 distribution (this was not the case inSurrey or Kent, where additional persistent sub-networkshad not been colonized between 1982 and 2000). In suchcircumstances, population translocations could play a partin facilitating species range shifts (Hoegh-Guldberg et al.2008), and approaches similar to that taken here couldassist in identifying habitat networks where species intro-ductions would be more likely to result in range expansions.There is also a message for projects aiming to restore habitatconnectivity. The climate-dependent nature of habitatassociations may result in climate-driven changes (increasesand decreases) to habitat availability which are much largerthan those brought about by conservation programmes forparticular species and landscapes. Therefore, such schemesneed to be carefully targeted if they are to achieve realincreases in species’ expansion rates.ConclusionsThe majority of species across the world exhibit very smallareas of occupancy (Gaston 1994, Kunin and Gaston1997), as exemplified by the many butterfly species thatare restricted to a few tens of km2or less in Britain (Cowleyet al. 1999). Many, and probably most, such localizedspecies perceive the environment as heterogeneous, withsmall areas of suitable habitat surrounded by much largerareas which are unsuitable for reproduction (Sobero ´n pers.comm.). Species of this type shift their distributions bycolonizing from one patch of suitable habitat to another,rather than expanding as a continuous front. Even if suchspecies do not exhibit metapopulation dynamics at equili-brium, they are likely to do so during climate change, whichis expected to render some existing habitats unsuitable andother previously-unsuitable locations available for coloniza-tion. Thus, metapopulation models could provide a suitableframework for the analysis of range shifts by many habitatspecialist species. Our research on the butterfly Hesperiacomma shows how metapopulation models can linkfine-scale changes in habitat use to large-scale changes inspecies distributions. The results of long-term metapopula-tion simulations show how metapopulations in different80S P E C I A LI S S U E